Capping off an improbable rise from the Boston City Council to the threshold of Congress, Ayanna Pressley made history Tuesday night by defeating Representative Michael E. Capuano in a closely fought Democratic primary. The resounding victory clears the way for Pressley, whose chances were discounted when she decided to take on a 20-year incumbent, to become the first black woman ever to represent Massachusetts in Congress.

It’s a milestone to behold, especially given the obstacles she overcame. Pressley went into the race with fewer resources and without the blessing of the Democratic establishment. Boston Mayor Marty Walsh, former governor Deval Patrick, and even some of her council colleagues quickly closed ranks behind Capuano. The two had few concrete policy disagreements, and supporters of Capuano pointed to his long voting record as a reliable supporter of progressive causes.

But Pressley — who became the first woman of color elected to the city council and then compiled an impressive record reforming the hidebound liquor licensing system — convinced voters to expect more. She made the case that the Seventh Congressional District — which the Legislature drew specifically as a majority-minority district — needed a representative with a more intimate understanding of its needs. Capuano, a former Somerville mayor, brought home billions from Washington. Pressley convinced voters that it was just as important what a representative brought to Washington.

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And her victory carries an unmistakable message, amplified by Tuesday’s other electoral upsets further down the ticket: Massachusetts politics is changing. Candidates aren’t willing to wait their turn anymore. The power of incumbency is weakening.

Pressley’s insurgent campaign should be a wake-up call to the old guard. Her victory is a political earthquake, and there will be aftershocks. New political alliances have been formed, anchored by a new guard of female politicians who stood by Pressley, notably Attorney General Maura Healey and Boston City Councilor Michelle Wu.

While much of the attention has been on Pressley, she was only the most prominent of challengers who sought to unseat incumbents. First-time candidates performed strongly in State House races. Boston emergency room doctor Jon Santiago beat longtime Representative Byron Rushing and newcomer Nika Elugardo ousted Representative Jeffrey Sanchez.

Some of the ferment might reflect special circumstances — namely, the grassroots anger at the Trump administration. But it also appears to confirm a trend.

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No, not every incumbent should go — this editorial page endorsed many of them for reelection. But every voter benefits from competition; just by running, challengers help hold incumbents accountable.

Let’s see more of this in the Boston City Council elections next year, and in 2020. Turning competition from the exception into the rule would help foster trust in city halls and on Beacon Hill, whose insularity goes hand-in-hand with uncontested elections.

The new reality — that officeholders should expect challengers — should encourage politicians to stay on their toes and take nothing for granted. At the same time, the success of challengers like Pressley should entice anyone who’d ever contemplated a primary challenge in Massachusetts.

Run. It’s a new world, and you don’t have to wait. Just ask Ayanna Pressley.